I first heard about Dau five years ago, and have been figuratively banging on the door ever since, to no avail. Two weeks ago, the door opened – literally. I am invited to a mansion block on London’s Piccadilly where, behind an anonymous black door, lies another world. The lobby is dimly, atmospherically lit – a room out of a David Lynch movie. A man in a 1940s-style suit and hat stands pulling back the curtain and peeking on to the street – except he turns out to be a mannequin. A bald security guard with a Russian accent takes my signature and photograph. Photography and recording are forbidden. A friendly American woman guides me through labyrinthine corridors painted black and red and grey, the walls plastered with Soviet-era posters and photos. There are more mannequins in vintage costumes in startling places: hanging from a chandelier, kneeling and kissing a Soviet flag. There is a large mannequin workshop. There are modern editing suites behind frosted glass walls. There is a restaurant serving Georgian cuisine, and a function room in 1940s-style decor. The sound of a dog barking carries down the corridor. “That’s a real dog, by the way,” my guide says. You can’t be too sure.

I am here to watch some films. Dau is a film project, or at least, it began as one. Now, nobody knows how to describe it. It has been called a “Stalinist Truman Show”, a serious anthropological experiment, even a “Soviet Love Island”. In scale, it has been compared to infamous movie shoots that spiralled out of control, such as Francis Ford Coppola’s Apocalypse Now. But Dau also has echoes of the Stanford prison experiment – in which studentslost sight of the fact that they were playing a role, acting as guards and prisoners rather too enthusiastically. Stanford’s experiment had to be shut down after six days; Dau’s continued for three years.

Dau began in 2005 as a conventional biopic of a real-life Russian scientist named Lev Landau. Landau worked with pioneering quantum physicists such as Niels Bohr, had a hand in the Soviet atomic weapons programme, and won a Nobel prize in 1962. He took an experimental approach to sex and drugs as well as science, living by his belief that marriage should be no impediment to sexual freedom (his wife, Kora, was less enthusiastic). The film’s director was Ilya Khrzhanovsky, a then 29-year-old Russian with just one previous work to his name – a challenging, hallucinatory drama titled 4, which had won considerable acclaim. The Greek-born conductor Teodor Currentzis was cast as the lead, simply named Dau. Non-actors made up the majority of the huge cast – 400 principal actors and 10,000 extras.

The central location of Dau was a Soviet research institute, inspired by the secret facility in which Landau lived and worked from the late 30s until his death in 1968. About a year into shooting, in 2009, a gigantic set, the size of two football pitches, was constructed at an abandoned swimming baths in Kharkiv, north-east Ukraine. But when it was completed, Khrzhanovsky abandoned the idea of finishing the film to focus exclusively on his replica institute, now also known as Dau.

It became less a film set than a parallel world: a functioning mini-state, stuck in the mid-20th century and sealed off from the modern world. Dau was populated with hundreds of extras, or “participants”, who lived as faithfully as possible as Soviet citizens. The period authenticity was obsessive, covering clothes (even down to the underwear), hairstyles, food packaging, cigarette brands. What is more, time moved forwards inside Dau, from 1938 to 1968, so all the period detail was continually updated. Participants were paid in Russian roubles, which could be spent on set. (Some of the older extras, playing caretakers, smuggled in their own roubles saved from the Soviet era and tried to spend them; they were reprimanded for using “fake” money.)

Participants were paid in Russian roubles, which could be spent on set. Photograph: Phenomen IP, 2019

Dau’s cast of non-actors was drawn from a nationwide database of auditionees compiled by the producers. The number of videotaped auditions fluctuates between 210,000 and 392,000, depending on who you ask; many of Dau’s “official” statistics are all but unverifiable. Roles were played by people who had played those roles for real: cleaners, waitresses, academics, party officials, shamans, artists; even, sometimes, criminals and real-life neo-Nazis, the latter of whom brought the project to a close by helping to destroy the set in “1968”. Scientists who visited or took up temporary residency there included Nobel-prize-winning physicist David Gross, neuroscientist James Fallon, and Harvard maths professor Shing-Tung Yau. The artists included Marina Abramović, Carsten Höller and theatre director Peter Sellars.

The weirdest thing was, hardly any of this was actually filmed. This was not a giant Big Brother house; there were no hidden cameras. A single cinematographer – German veteran Jürgen Jürges – roamed the set with a three-person crew. Between 2009 and 2011, he filmed 700 hours of footage – only a fraction of the duration of the “experiment”. The rest of the time, people apparently went about their Soviet business, unobserved.

The Scottish producer Eddie Dick got a taste of Dau in 2011, when he went to Ukraine to discuss a potential collaboration between Khrzhanovsky and director Nicolas Roeg. The induction process was meticulous, he tells me. He and his colleague were given a crib sheet of facts about 1953 – the current year inside Dau. They were dressed in period costume; Dick’s hair was cut; new spectacles with more period-appropriate frames were quickly ordered from a local optician. They were given “passports” and some (real) roubles. On the threshold of the set, their papers were checked by guards and they were interrogated about the purpose of their visit.

“I wasn’t sure if everybody started doing things when we appeared, but certainly everybody was doing what they were supposed to be doing,” Dick says. Armed guards were marching up and down. People were eating in the cafe; three or four scientists were conducting experiments in the laboratory with electromagnetic guns. The journalists were preparing the newspaper of the day. The architects were poring over a plan of a Kharkiv of the future – as in, the 1960s. He even visited Dau’s apartment and met his wife, Nora, played by the Russian actor Radmila Shchyogoleva. “A tiny babushka opened the door. Nora floated downstairs in an elegant dress and had tea with us for half an hour. She was in character the whole time. The illusion was kept up by everyone and everything,” Dick says. “We never broke the rules, either. I never turned to anyone and said, ‘Come on. What the fuck’s going on here?’ You found yourself mesmerised by the facade, and we just went along with it.” But throughout Dick’s visit, he saw no filming taking place.

Some people allegedly moved to Ukraine and lived at Dau for months, even years, eating, working and sleeping on the set. Others came and went quickly, convinced that this recreation of a totalitarian state had morphed into something genuinely oppressive and dark. Rumours began to circulate that Khrzhanovsky was as interested in sex and power as he was in art. Rather than Coppola’s shoot of Apocalypse Now, Dau began to sound like the heart of darkness itself: an entire world cut off from civilisation, living by its own rules, with Khrzhanovsky as its Colonel Kurtz figure, a film-maker gone rogue.

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The reason Dau’s door opened to me is that, nearly 15 years after its inception, the project is finally ready to show to the public. Three immersive exhibitions are planned for this year, in Paris, London, then Berlin. Before the opening, I am promised an interview with Khrzhanovsky in Paris. First, though, I am required to watch at least four of the 13 feature films that have been edited from 700 hours of footage from the institute. (More is planned: a TV series, documentaries, and, at some point, a cinema release for the original Dau movie.)

The first three films I am shown are a trilogy centring on Nora. The films are in Russian with no subtitles, but through an earpiece a monotone voice narrates the English translation. In the first, Nora’s mother comes to visit her at the institute – which looks less like a genuine Soviet facility than a postmodern fantasia based on one. In the second film, Dau (Currentzis) is visited by his former lover, a beautiful Greek woman named Maria. Nora is away, but then returns unexpectedly.

The third film, set more than a decade later, is more startling. Dau has considerably aged and is almost bedridden (the real Lev Landau had a debilitating car accident in 1962; he died in 1968). His son Denis has grown up into an eccentric man-child. Nora is bored and lonely. There is incest, and prolonged scenes of graphic, unsimulated sex. In the fourth film, the institute’s caretakers get drunk on vodka. An old lady vomits copiously. Then, for the bulk of the film, two men, Sasha and Valera, embark on a night of drunken passion that swings between tenderness, clumsy sex and physical and verbal abuse.

Lev Landau lectures students in a scene from the original film, for which Khrzhanovsky plans a cinema release. Photograph: Phenomen IP, 2019

The films are both exhilarating and boring. There are long, rambling dialogue scenes with very few cuts (Jürges shoots with a single camera). But there is an authentic emotional rawness and intensity to the drama. The experience is closer to watching a documentary, or perhaps a Danish Dogme film, like Lars von Trier’s The Idiots.

The true scale of the project becomes apparent when I am shown a 10-minute trailer for the original, still unfinished Dau movie, which looks appetisingly epic: vast crowds, 1930s street scenes, a huge replica Soviet propeller plane. There are countless striking images: a masked ball, a church bell crashing to the ground, piles of rotten cabbages, women smashing up clay rabbits on a production line.

Time moved forward on set from ‘1938’ to ‘1968’. Here extras play shopkeepers in ‘1952’. Photograph: Phenomen IP, 2019

I am also shown the digital component of the project. In Paris, visitors will be able to explore hours of footage from within custom-built booths, like confession boxes. My screen is divided into a grid of 16 sub-screens, each playing a random scene, like a bank of surveillance cameras. Click on one and it becomes full-screen. Supplementary information is available on each character in any scene: biographies, photo galleries, documentation such as passports and letters.

Scanning this brings up a bewildering panoply of images: Abramović (in 50s costume) undergoing a purification ritual by a shaman; Fallon discussing capitalism at a dinner party; Dau standing over a naked man and woman having sex (there is always at least one screen playing sexual content). An official abusing a tearful, naked woman in a prison cell; shockingly, he forces her to drink cognac, then to put the bottle in her vagina. Nothing looks simulated.

I recognise people from the movies I have seen: a bearded scientist who insulted Dau’s son as “clinically retarded” is now discussing a recent ayahuasca experiment. Elsewhere, one of the waitresses from the institute’s cafe is standing on a table with two men in their underpants at a party. They all begin throwing knives at an abstract painting. “That’s what I call contemporary art!” one of them exclaims. “Now we can put this up in a gallery.”

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Dau’s Paris launch took place this week in the half-renovated spaces of the Théâtre du Châtelet and Théâtre de la Ville. “I’m absolutely, 100% sure that what we’re experiencing here will be in the history books of Paris, of art history, of scientific innovation, of cinema history,” says Ruth Mackenzie, artistic director of Théâtre du Châtelet. The former director of the London 2012 Olympics cultural programme, Mackenzie has seen some grand cultural spectacles in her time, but describes Dau as a once-in-a-generation “gamechanger”.

Others are more sceptical. “I’m convinced the reason the project has been active for so long has nothing to do with creative reasons, but that it allows Khrzhanovsky to live a luxurious and tyrannical lifestyle, lording it over dozens, if not hundreds, of people on a Russian oligarch’s dime,” says one critic, who asked not to be named.

Dau with his on-screen son Denis and wife Nora, in ‘1968’, the year Dau died . Photograph: Phenomen IP, 2019

The “oligarch” is no secret: he is a Russian entrepreneur named Sergei Adoniev, who became a Bulgarian citizen in 2008. One estimate puts his wealth at $800m, mostly made in the Russian telecommunications business. He also funds Russia’s opposition newspaper Novaya Gazeta, and largely devotes his time and wealth to cultural and philanthropic causes. But this week Bulgaria rescinded Adoniev’s citizenship; a ministry of justice spokeswoman said this followed notification that he “had been convicted in the United States 20 years ago on fraud charges”.